The present invention relates to a test pattern for monitoring variations of critical dimensions (CDs) of patterns produced during the fabrication of semiconductor devices, and in particular an improved test pattern which allows the critical dimensions to be easily monitored by a microscope.
Conventionally, semiconductor devices have multiple layered structures of 4 to 5 layers or sometimes up to 20 layers on a semiconductor wafer. Each of such layers is produced on the wafer by the process of growing or depositing a silicon oxide, a silicon nitride, a PSG, a BPSG, or the like, and then etched to desired design patterns by the photolithography technology. Therefore, the integration of the layers results in each individual chip on the wafer. However, since dimensions of patterns formed on the wafer greatly affect the operative characteristics of resulting devices such as the yield and speed thereof, the dimensions during the fabrication of semiconductor devices should be controlled as accurately as possible so as to substantially render the dimensions the same as those of their designed patterns.
Therefore, a certain process step requires to measure widths or critical dimensions (CDs) of patterns produced on the same layer during the fabrication of a semiconductor device. However, because it is very inconvenient to respectively measure the dimensions of patterns by using a dimension measuring instrument, a special test pattern to monitor changes of the critical dimensions (hereinafter referred as to "CD bar".) has been employed in scribing regions or regions where any device element patterns are not formed. Moreover, as dimensions on semiconductor devices become smaller, the CD bar becomes a more convenient way to measure changes of CDs because it enables the position of CD bar to be read by using only a microscope. That is, since various patterns formed in the same layer are produced through the same etching and photolithographic method even though each CD of the patterns is different, the variation of the CDs are equally kept up throughout all the patterns. Accordingly, it becomes possible to detect the incremental increase or the decremental decrease of each widths of patterns with the CD bar.
The CD bars as aforementioned are divided into two categories in the art. One of the two is a CD bar to measure the CD by using a special measuring machine and the other is a CD bar having a special pattern to measure the CD only by an eye of observer using a microscope. The former CD bar can serve a precise measurement but makes the measurement itself complicated. The latter CD bar makes the measurement easy though its accuracy falls below. Since the fabrication of semiconductor devices is to form in sequence a plurality of various layers as mentioned above, it is very complicated and time-consuming work to measure each CD by the special machine after each pattern is once formed. Thus, the eye-measurable CD bar using a microscope generally prevails in the art, in which the special patterns of test geometry are disposed in a proper arrangement so that ony dimensions in either horizontal sides or vertical sides thereof are different from one another bythe minimum CD variation value for measurement desired in designing. The use of an eye measurable CD bar makes it easy to monitor the variation of total CDs according to each CD changing in the fabrication of the semiconductor device.
Referring to FIG. 1, rectangular patterns 10 through 17 represent eye-measurable CD bars of currently practiced are showing various changes of CDs. Each horizontal side of upper rectangular patterns 10, 12, 14 and 16 or lower rectangular patterns 11, 13, 15 and 17 has sequentially a length increased by the minimum CD variation value (hereinafter assumed to be 0.1.mu.m). Also, each lower rectangular pattern is positioned at a lower spaced region between the two adjacent spaced upper rectangular patterns. Therefore, the variation of the CD could be detected by reading an indication numeral when both vertical sides of upper and lower rectangular patterns meet in a straight line. FIG. 1A represents a case that there is no change of the CD. One pair of vertical sides of a lower rectangle 11 and a upper rectangle 12 are set in a single straight line at an indication numeral "0", which shows the fact that there is no change of CD. FIG. 1B represents the patterns when the existing CDs decrease in size by 0.2.mu.m, wherein the rectangles of FIG. 1A decrease by 0.1.mu.m in all directions so that their horizontal and vertical sides eventually decrease by 0.2.mu.m in each length. Thus, a pair of vertical sides of a lower rectangle 13' and a upper rectangle 14' are set in single straight line at an indication numer "2", which shows the fact that the CDs decreased by 0.2.mu.m. As aforementioned, FIG. 1C shows that the CDs decreased by 0.3.mu.m and FIG. 1D shows that the CDs increased by 0.1.mu.m. Therefore, it is possible to visibly measure the variations of CDs by each 0.1.mu.m unit. But, because there is no reference line which makes an observer clearly decide whether a pair of vertical sides of an upper-row rectangle and a lower-row rectangle are set in a single straight line or not, there often arises a considerable observation error. To supplement this defect, the shape near the center line of the patterns has changed in a dotted portion shown in FIG. 1A, wherein the correponding corners of the upper and lower rectangular patterns are made so as to be in just contact with each other when the two vertical sides of said rectangular forms are set in a straight line. But it is impossible to keep up therein a sharp cornere angle due to the limitation of photolithographic technology itself in a fabrication process and further difficult to attain a good effect therein because there forms a portion that the etching is not performed between the two corner points.
Referring to FIG. 2 which shows CD bars called Murray Daggers of other currently practiced art, right triangles shaped in steps are illustrated. The size of those incremental steps is assumed to be 0.1.mu.m. Therefore, whenever the CD decremental decrease of patterns produced on the wafer is 0.1.mu.m, the tip step of the triangles disappears one by one. The resulting tip end thereof indicates some marks or numerals next to the triangle so that the CD variation can be monitored. Therefore, by reading those indications, it is possible to measure the CD decreasing widths. Referring to FIG. 2A, the decremental decrease of the CD variation is illustrated to be within 0.1.mu.m. FIG. 2B shows a case which that of the CD variation is between 0.1 to 0.2.mu.m. FIG. 2C shows a case between 0.3 to 0.4.mu.m. The CD bar of Murray Daggers can be employed in case CDs decrease, but it can not be used in case CDs increase. The reason is that though the width of incremental increase of the tip step changes, it gives no changes in length as shown in FIG. 2D.
The measuring capability of Murray Daggers is tied to the photographic technology itself. However, photographic technologies in currently wide use make it difficult to produce accurate photo mask patterns below 0.5.mu.m. Therefore, the CD bar of Murray Daggers makes it useful only when widths of the tip step, that is, the minimum CD variation value in designing are above 0.5.mu.m.